Antarctic Crustal Profile (ACRUP) Seismic Project

Cruise Report for R/V Explora EXPL 93-1

Summary

In January and February 1994, the U.S. Geological Survey conducted seismic investigations in the Ross Sea and adjacent Transantarctic Mountains, as part of the international Antarctic Crustal Profile (ACRUP) project to map the crustal structure of the West Antarctic RAft system. The ACRUP project was conducted under the logistic coordination of the Italian National Antarctic Research Program (PNRA), with planned involvement of six research organizations from four countries including USA, Germany, Italy, and Japan. The U.S. Geological Survey participated with the logistic and science-grant assistance of the U.S. Antarctic Program, Office of Polar Programs in the National Science Foundation.

The ACRUP project had a land component, coordinated from Start Nunatak Camp near Tetra Nova Station (Italy), and a marine component, conducted aboard the R/V OGS EXPLORA (Italy). The purpose of this preliminary report is to give a general overview of the ACRUP project and describe the activities and contributions of the U.S. Geological Survey during the field operations. Complete descriptions of the land and marine field operations for the ACRUP project are reported by the Italian National Antarctic Research Program (Bacigalupi and Ramorino, 1994).

The objective of the ACRUP project is to study the structure of the crust and upper mantle of the Ross Sea region using geophysical measurements along onshore and offshore transects. The Ross Sea is part of the West Antarctic Rift system (WRS). The WRS has a 2500 to 5000-m-high rift shoulder (Transantarctic Mountains (TAM)) on the west, and is underlain in the Ross Sea by numerous large rift basins now filled with thick sedimentary sections. A deep sedimentary basin, the Victoria Land basin, and an active rift (Terror rift) are mapped along the western side of the Ross Sea adjacent to the TAM. The rift structures of the Ross Sea are believed to have formed initially with the Mesozoic breakup of Gondwana, and later with late Mesozoic and Cenozoic extension across the Ross Sea and uplift of the Transantarctic Mountains. The age of the Mesozoic and Cenozoic rifting is poorly known because rocks from deep within the rift basins have not been sampled . Also, the crustal structure and mechanism(s) of rifting are not known because few refraction data exist, although many models for the evolution of the WRS and uplift of the TAM have been proposed. The ACRUP experiments are designed to determine deepcrustal structures in the Ross Sea region to help resolve the mechanisms, and relative timings, of rifting events.

Initially, two ACRUP transects were planned for the 1993/94 field season: 1) an E-W transect along latitude 76 degrees South, extending about 400 kilometers from the polar plateau of the Transantarctic Mountains to about 200 kilometers offshore in the Ross Sea; this transect would cross the flank of the West Antarctic Rift system (i.e. boundary between East and West Antarctica along the TAM) and the adjacent offshore Victoria Land basin and Tenor rift; and 2) a 200kilometers long, N-S transect along the axis of the Victoria Land Basin (longitude 164 degrees West); this transect would examine the crustal structure of the 14 kilometers thick sedimentary section and underlying thin(?) crust of the basin and active Tenor rift.

Unusually bad ice conditions were encountered in the western Ross Sea during January and February 1994. More than 80% of the planned operating areas were covered by heavy pack ice. Consequently, the locations of offshore crustal transects had to be changed, and the planned onshore-offshore long-range seismic experiment could not be done. In addition, the Japanese ship R/V akurei-Maru was unable to join the ACRUP project at the last minute due to the bad ice conditions in their other working area of the Ross Sea, and due to heavy pack ice across the entrance to the Ross Sea. The offshore ACRUP transects were subsequently relocated to the southern Ross Sea to cross the deep rift structures beneath these areas. Two transects were done, back-to-back, yielding a continuous transect that extends nearly 650 kilometers across about 70% of the southern Ross Sea along latitude 77 degrees 06 minutes South from near Ross Island (168 degrees East) to the middle of the Eastern basin (longitude 167 degrees West). The onshore part of the ACRUP project was done as planned, and several additional seismic stations were placed on the fast ice along a seaward extension of the profile out to about 30 kilometers from shore, over the Victoria Land basin.

Many different types of geophysical surveys were conducted during the onshore and offshore parts of the ACRUP field program. Scientists from each of the three countries participated and contributed equipment for use in the field program. The overall logistics for the operation were coordinated and principally sponsored by Italy. The tables (in the data section) list the general contributions to the science program by each country and organization; the contributing organizations; and participants in the ACRUP field work.

Cruise Narrative

The following sections of the report give details for the field operations for the offshore and onshore parts of the USGS involvement in the ACRUP project. The offshore sections are outlined in greater detail, because the principal scientist and first author of this report participated in the EXPLORA cruise, and was not directly involved in the daily activities of the onshore field work. The reader is directed to the comprehensive field-report, now in preparation by the Italian National Program, for greater detail of the onshore and offshore parts of the experiment. Scientific results will be presented in later publications after data processing and analysis.

Transit to Antarctica

The U.S. scientific staff for the offshore part of the 1994 Antarctic Crustal Profile (ACRUP) project (i.e. Dr. Alan Cooper and Mr. Gregory Miller) departed the United States for Antarctica on January 1. After stopping in Christchurch for Antarctic clothing issue, the staff continued to Dunedin, New Zealand to prepare for arrival of the R/V OGS EXPLORA. Upon arrival of the ship, the staff loaded the 5800 pounds of US Geological Survey Ocean Bottom Seismometer (OBS) equipment aboard during the period of January 7-10, 1994. On January 10, the ship departed for the Ross Sea, Antarctica, arriving at the entrance to the sea on January 16. Due to heavy ice conditions, the entrance was closed by up to 9/10 pack ice, and the EXPLORA could not enter. By good fortune, the USCG POLAR SEA was escorting the tanker-ship MATHESSIAN into the Ross Sea, and provided ice-breaking assistance for the EXPLORA to enter the Ross Sea on January 17 and 18. The EXPLORA then proceeded to the Italian Terra Nova Station, through heavy pack ice, to offload supplies. On January 20 EXPLORA left Tetra Nova Station and proceeded to Franklin Island area, arriving on January 21 to deploy equipment for commencing seismic transects.

Seismic Reflection Operations

Due to heavy ice cover of the western Ross Sea, from the coast to Franklin Island, it was not possible to deploy OBS equipment across nearly all of the Victoria Land basin, E-W along the planned ACRUP- 1 transect of the Transantarctic Mountains and westernmost Ross Sea along 760 S. Open water east of Franklin Island allowed four OBSs (2 from Italy and 2 from Germany) to be deployed at the far east end of the ACRUP transect at 10km intervals. The westernmost OBS was deployed about 1 km from the edge of the ice pack. Thereafter, the EXPLORA deployed seismic gear to begin an E-W multichannel seismic-reflection (MCS) profile across the Ross Sea. The equipment and parameters used included:

36-airgun array (one string of 17 guns and one string of 19 guns with a total volume of 71.5 liters) marine gradiometer seagravimeter

12-kiloHertz bathymetry

Integrated GPS navigation system

Following deployment and testing of the MCS equipment, which required about 29 hours, a sequence of 40 airgun shots at 120 sec intervals were made while the ship was circling to begin the MCS profile. These shots were intended to be recorded by land seismic stations along the ACRUP transect in the Transantarctic Mountains. The EXPLORA recorded an E-W MCS profile between January 24 and January 27. Thereafter, a second MCS profile was recorded heading NW until January 28. The two MCS profiles were recorded for the University of Trieste as part of the Italian Antarctic Research Program.

Offshore ACRUP Transects

From January 28 to February 17, the EXPLORA conducted OBS operations as part of the international ACRUP project. Two offshore-only transects were recorded, each about 300 kilometers long. The two transects, end to end, with a center-point near 180 degrees longitude, extend from the eastern flank of the Victoria Land basin (directly north of Ross Island) to the middle of the Eastern basin (see Line 1 plot in the data section). The ACRUP transect in the eastern Ross Sea was recorded first, followed by that in the western Ross Sea. The complete transect is located directly over previously recorded MCS seismic line USGS 404 (USA) and BGR 2 (Germany).

ACRUP-ERS Transect - Eastern Ross Sea

The ACRUP-ERS transect commenced on January 30 directly following a bad storm and long transit from the end-position of MCS profile 2. Twenty-six OBS stations (6 from USA, 6 from Italy, and 14 from Germany) were deployed along the 300-kilometers-long transect with a station spacing of 10 kilometers, and with gaps of 30 and 40 kilometers at the west and east ends of the transect, respectively. After OBS deployment, the full complement of geophysical gear was deployed (see above) in about 6 hours. A 1500 m-long-streamer (120 channels) was used instead of the 3000-m-long streamer. Shooting of the ACRUP-ERS transect began on January 31 and ended on February 2. The first phase of OBS recovery, during which all but 5 OBSs were recovered, started February 2 and ended on February 4. On February 5, necessary repairs and tests were made to ship, MCS, magnetometer, and side-scan sonar equipment. The final phase of OBS recovery was completed on February 6, during which all but 2 OBS (both from Germany) were recovered.

Following completion of the ACRUP-ERS transect, the EXPLORA transited to Franklin Island to
recover the four OBSs set out earlier. On February 7, three OBS were recovered,
but the fourth OBS (from Italy) could not be reached because the ice-front
had moved 7 kilometers east and now covered the OBS station. Thereafter, EXPLORA
transited to a point about 30 miles north of Ross Island to be refueled
from the ship ITALICA on February 8.

ACRUP-WRS Transect - Western Ross Sea

The ACRUP-WRS transect commenced in the evening of February 8 with OBS deployment about 6 kilometers
from the edge of heavy brash ice north of Mt. Terror on Ross Island. Thirty
OBS stations (6 from USA, 7 from Italy, and 17 from Germany) were set
out at 10-kilometers intervals along the E-W transect, which followed a latitude
of about 77 06' S. The position of several OBS stations had to be moved
2 to 3 miles north of the transect line to avoid the advanced position of the Ross Ice Shelf (17 kilometers north of charted position), and the concentrations of heavy brash ice near the advanced Ice Shelf.

OBS deployment was completed on February 9. Prior to shooting the OBS line, Scott Base was contacted
to determine the possibility of using the downhole seismometers near Lake
Vanda in the Dry Valleys (i.e. about 77 degrees 35 minutes South and 161 degrees 30 minutes East) to attempt
to record the offshore seismic shots. The principal contractor for the seismometers,
Allied Signal Company in Albequerque, New Mexico, said that this would not
be possible on short notice because 3-4 days were required to recalibrate
the equipment for the 0-25 Hz frequency range needed for the experiment.

In preparation for shooting the OBS transect, it was discovered that extreme cold weather
and ice had caused several problems (frozen winches, snapped air hoses,
and airguns filled with ice), causing an 8 hour delay for repairs. The
shooting phase began on February 10, and was done with the same geophysical
equipment as used on the eastern Ross Sea transect (see above). During
the shooting of the first half of the ACRUP-WRS profile, the OBS data
over more than one-half of the transect were corrupted by external seismic
noise, which originated from the nearby (i.e. within 0 to 10 miles of
the transect) shooting of seismic-reflection equipment on the R/V N.B.
PALMER. The PALMER arrived at the ACRUP transect site 2 days earlier than
previously agreed. After shooting along a converging track to a point
directly on top of the ACRUP profile, PALMER stopped shooting their airgun.
PALMER then did coring operations close to the ACRUP profile for 4 hours
and resumed their seismic shooting away from the ACRUP area as EXPLORA
circled for 5-6 hours. Shooting of the ACRUP-WRS transect was completed
without further problems on February 12, at a point about 40 kilometers west of
Station 101.

The OBS recovery phase began on February 12, following about 7 hours of seismic-streamer repairs. High velocity (30 kilometers) catabatic winds and resulting bad sea conditions
near Mr. Erebus caused great difficulty with recovery of the first eight
OBS stations. Thereafter, conditions improved and OBS recovery was completed
in the early morning of February 17 following a delay of more than a day
to repair and test ship and science equipment. All but seven OBSs were
recovered, with losses of 5 OBS by Germany and 2 OBS's by Italy. All
USGS OBS were recovered. EXPLORA then proceeded to McMurdo Station to disembark
four members of the science staff (Cooper, 1 Italian, 2 Germans) to fly
to New Zealand. EXPLORA remained at McMurdo Station for a 4-hour visit
and guided tours by US Antarctic Program staff before departing for New
Zealand. Greg Miller remained on the EXPLORA to insure the well being
of the USGS OBS equipment during the long transit. The EXPLORA arrived
in New Zealand on March 3, at which time the USGS OBS equipment was offloaded
and shipped back to the US Throughout the 1993/94 field season, the US
Antarctic Program was highly responsive to requests from the EXPLORA staff
(forwarded by Alan Cooper) for assistance with logistic support for the
ACRUP science program. Such help included

Assistance by the USCG POLAR SEA in getting EXPLORA into the Ross Sea and in offering to pickup US and foreign scientists on EXPLORA and transport them to McMurdo Station (NB: the offer of transport help was not used because ice conditions improved at the last moment).

Regular assistance with providing ice and weather reports to the EXPLORA in the Ross Sea.

Providing cold-weather clothing for foreign scientists from EXPLORA and flying them (with Cooper) to New Zealand.

Hosting EXPLORA at McMurdo Station.

Many other smaller requests throughout the field season.

Without such timely cooperation, the planned ACRUP science program could not have been done as safely, successfully, and effectively as accomplished, especially in the offshore areas.

Cruise Data

Two ACRUP transects were recorded, each with several types of geophysical data (see cruise narrative above). Ocean bottom seismometer (OBS) data were the principal data recorded offshore by the USGS. The following graphics show the deployment locations for the USGS OBS's for both deployments. The line drawn on each graphic is the navigation track of the shooting ship for the line shot specifically for OBS recording. These are interactive displays. Click the mouse button on an OBS location, and a sample data section will be displayed with the details of each deployment site. The data displayed are raw record sections of the vertical geophone from each instrument. The first deployment, while a success operationally, was a near disaster for data recovery. This operation required a longer record window than had previously been available, so a new operating system had to be installed in the OBS datalogger's. In transferring the data-aquisition software to the new system, one bug was not caught. The timing routines were storing a variable where the new operating system saved the hard disk mapping information. As a result, data was stored all over the disk in what appeared to be random locations. With some work, the data was recoverable but, after only a third of the line was shot, the datalogger thought its disk was full and shut down.